The objective of this project is to characterize dose-response relationships and toxicity of hexachlorocyclopentadiene administered by inhalation to male and female rats and mice in 90-day subchronic and 2-year chronic studies. Animals will be exposed for 6 hours per day, 5 days per week for 13 and 104 weeks. At the end of each study, all animals will be given gross and microscopic histopathologic evaluations. Hematology, urinalysis, and clinical chemistries analyses will be performed on all animals in the 13-week studies. Hematology and clinical pathology will be evaluated on all animals sacrificed at the 65th week of the chronic study. The results of the 13-week study will be used by NTP to establish doses to be used in the chronic study. The results of the 2-year chronic study will be used by NTP to determine toxicologic and carcinogenic effects of inhalation administration of hexachlorocyclopentadiene.